While I was enjoying Start's excellent turntable pictorial on FB, I was fascinated by the picture of the table at Albion, where the left most queen post has evidently been forced away from where it was supposed to be.The comment was that the Wiscassett turntable had an extra tie rod to prevent this.That is a bit confusing to me. I would have expected the tension in the rod to force the posts toward the middle, hence the angling of the posts outward to counteract that tendency.Does anyone know why this might happen? Might it have something to do with the stresses on the table when the engine is run on or off?More importantly, should we understand that before we get started on the turntable?

Well, as a matter of fact, I do know. The queen post angle if set at the optimal angle for the outer truss rods would have a resultant force that is directly in line with the queen post --pure compression. Unfortunately the optimal angle for the inner truss rods is different, since the inner rod is at a steeper angle down to its anchor. Thus one has to compromise; the best angle for the taller queen post is not the same as that for the shorter one.

The Portland Co. drawing shows that they split the difference. The angle is set so that the upper tension rod tends to push the tall queen post outward, and push the short post inwards. This requires the additional tension rod as shown in the pictures to resist the outward force, and the compression beam that you see at the base of the queen posts to resist the inward force on the short queen posts.

Since I did a stress analysis on the turntable, I can state that actual amount of outward or inward force is not that great - only a few hundred pounds. The load on the inner truss rod turns out to be much higher than the outer, and when the locomotive is halfway on the turntable, the outer truss rod tension drops a lot, and the inner truss rod tension is at its highest. In fact in every load case I analyzed, the inner truss rod has the highest tension.

So, for our turntable, I chose the queen post angle to be a compromise as well, but biased slightly toward the optimal angle for the shorter queen post. The tension rod on the taller queen posts is absolutely required, and is in our design.

I didn't really consider making the queen post angles different. As you mentioned, the goal was to replicate the Wiscasset turntable, using the Portland Co. drawing as a reference. Since I found no real design/stress issues with making both posts the same angle, I saw no reason to consider anything else.

Not being a bridge design expert, I can't really comment on one design vs. another.

Overall, this turntable bridge is a very robust structure. The stresses in all the main elements are very low. The primary limiting factor was deflection, which was governed by the longitudinal beams. The truss elements function to keep the loads on the perimeter rail and wheels near zero once the locomotive is centered. In fact if you look at some of the historical photos, there is only a partial perimeter rail.

Finally, the bridge would still function without the truss, but it would be harder to turn.

For the stress analysis, I used the biggest WW&F locomotive as the design load --the 26 ton #6.

It was quite fun to look at the historical photos and analyze the changes from the Portland Co. drawing. Folks have noticed the tie rod between the queen posts so far. There are quite a few other differences, some of them subtle. Our turntable will be very much like the historical photos.

While the turntable hasn't been a secret, those of us involved have kept it largely under the radar, not wanting to distract from more current projects. Now that the turntable is at the forefront of activities for the coming year, I suppose it's time to shed some light on the project, it's design, and what's been done so far.

Two years ago Jason and Dave Crow were discussing ways of forming the ring rail, and spider rails. Dave got in touch with me to see if I would be interested in getting started on the patterns so that we'd be in a better position to get going on the turntable when the time was right. So he sent me a scan of the Portland Co. drawing, and I started to scale it to design the patterns. It just so happened that my dad was visiting me at school that weekend, and we broke out my copies of NGSV just to scope out the design. It wasn't more than an hour or two later that he was drawing things up in Autocad, trying to reconcile the pictures and the plans. Dave and Jason indicated that nobody had yet set upon designing it, so I talked my dad into taking it on.

During the winter and spring of 2014, I made the patterns for the center pivot, and the wheel which would be used for the spider bearing, and outer supports, and Harold worked through the design and the stress analysis. As he said previously, the whole thing will be and look very much like the original. For the structure of the turntable itself, there were very few compromises made, in order to maintain the integrity of such a recognizable feature of the original railway. However, there was at least one design flaw with the original Portland design which needed to be addressed. If you look at any pictures of the Wiscasset or Albion tables, you can see substantial blocking placed inboard of the cross beam which the inner truss rod is mounted to. The truss rod is trying to tilt that beam inwards, and the only thing holding it in place is the tension of the bolts holding it to the main beams. The blocking which the railway added was a divergence from the Portland Co. Plans, and was their effort to keep the cross beam from folding inwards, and you can see that it was only marginally successful. The mounting configuration of the outer truss rod is much more appropriate, in the way that it's load is directed through the main beams. In order to achieve a similar loading configuration for the inner truss rod, we designed a large bracket for the truss rod to pass through, and be tightened against. The bracket will be set into and bolted to the main beam, and butted against the offending crossbeam. This should shift the majority of the load onto the main beam, and prevent the cross beam from being rolled over. We designed the bracket to be a casting to be within the same spirit as the original designers might have done. My dad made the pattern for this bracket, and it was visible in the Facebook post from earlier this week, that sharp eyed viewers may have noticed was not in the original plans. All of these patterns were delivered to the museum in the summer of 2014, with a mess of #11 patterns as well.

My role with the turntable largely ended with the patterns. Since then Harold has been refining the design to suit, and creating part and assembly drawings. However we did spend a day last summer gathering up and counting all of the bridge washers on the campus. There were hundreds of them, and we have enough on hand to cover the ~500 which are needed for the turntable.

Now, we're both really looking forwards to the real work getting started. The project is now in the hands of the talented and dedicated on site crew.

Here's another couple of fun facts about the turntable: On each end there is a lantern bracket, about 3 feet from the end, to hang a lantern for night operations. There is also an angled socket on one side at each end to hold a turning pole.

The turning poles must either have gotten lost, or the force to turn the table got too high, because some historical photos show workers down in the pit pushing on the side of the bridge to turn it.

Based on the photos it seems like this turntable had much less of a traditional pit and more of what could be called a foundation ring to support the ring rail. In the photos you can see that the ballasted track comes to a level pretty high above the ring while in other places it falls away from it. If I recall correctly, this is similar to the way the SR&RL turntable in Phillips is installed.